This invention relates to novel photopolymerization sensitizers and to methods for their preparation comprising bonding benzoin derivatives into organopolysiloxanes.
Various photosensitive or photopolymerizable (hereafter to be called photosensitive) resins are conventionally known. Several photopolymerization sensitizers have been developed or proposed for the photopolymerization of these photosensitive resins. The required characteristics for these photopolymerization sensitizers are good sensitizing action and good miscibility with the photosensitive resins. If its miscibility with photosensitive resins is poor, the polymerization sensitizer will separate from the photosensitive resins and the desired sensitization action cannot then be adequately achieved. When the photosensitive resin polymerizes and hardens, nonuniformities are formed therein and the sensitizer is deposited or separated from the hardened photosensitive resin. In cases where photosensitive resins blended with the photopolymerization sensitizer are stored, long-term storage is a problem.
There are many types of conventionally known photosensitive silicone resins and photosensitive organopolysiloxanes and it is necessary to use photopolymerization sensitizers with these resins. For this purpose, sensitizers such as 2,6-dichloro-4-nitroaniline, 2,4-dinitrophenol and other aromatic compounds; Michler's ketone, benzophenone, benzoin, and other ketones; and benzoquinone and other quinones are known to be effective. However, these sensitizers have the drawbacks and disadvantages described above since they are poorly miscible with photosensitive organopolysiloxanes. Sensitizers which are miscible with photosensitive organopolysiloxanes have been proposed in Japanese Patent Sho Nos. 51 (1976)48794, 52(1977)50074 and 54(1979)44643. They are benzophenone derivatives and benzoin derivatives substituted with organosiloxy groups. These sensitizers are complementary materials because the benzophenone moiety has an ultraviolet absorption maximum at 365 nm and the benzoin moiety has an ultraviolet absorption maximum at 247 nm. However, they are not perfectly miscible with photosensitive organopolysiloxanes and they lack hydrolytic stability. Therefore, they have no practical use. This invention was achieved as the result of research by the present inventor in order to overcome these disadvantages of the conventional technologies.